5Computer aided microfiuorometric measurements have been used with both human and phage DNA to study the effects of the anticancer drugs cisplatin and/or 5-fluorouracil on the proofreading activity of the enzyme T4 polymerase. An in-vitro assay, that can measure the degree of drug binding and its effect on different classes of enzymes, for drug tolerant and drug sensitive patients, is being developed. Such as assay promises to be clinically useful in selecting optimal drug dose and for quantifying the synergistic effects of multiple drug and/or radiation therapies using the DNA of patients undergoing treatment. In addition, measurements of intramolecular lengths, angles and fluorescent density distributions, taken during agarose gel electrophoresis, have been used to quantify the elastic properties of the DNA molecule. DNA elasticity, thought to be important in transcriptional regulation and the control of development, is also being studied as a function of therapeutic treatment. Atomic force microscopy of oriented, 1 micron diameter, fibers of DNA is being developed, in parallel with the epifluorescence microscopy studies, to study how particular drugs influence the structural properties of DNA. For native DNA, atomic force microscopy images, detailing the molecular architecture within an oriented fiber, have revealed a regular array of twisted 50 nanometer wide ribbons of DNA. Cisplatin treatment markedly effects the geometry of these ribbons, and statistically significant differences can be measured as a function of drug dose.